1 Field of the Invention
The present invention relates to silicon wafer storage water and a silicon wafer storage method which are used for storing a silicon wafer in water, and particularly to silicon wafer storage water and a silicon wafer storage method which are used for storing a silicon wafer in water immediately after polishing.
2 Description of the Related Art
A process of producing a semiconductor wafer typically comprises the steps of slicing a monocrystalline ingot to obtain a thin disk-shaped wafer; chamfering the outer circumferencial portion of the thus-sliced wafer for prevention of cracking and chipping thereof; lapping the surface of the wafer to attain flatness; etching for eliminating mechanical damage that remains on the chamfered and lapped wafer; polishing the surface of the wafer to attain a mirrored surface; and cleaning the polished wafer to remove the polishing agent and foreign matter adhering thereto.
These are the main steps. In some cases, other steps such as heat treatment are additionally performed, or the sequence of the steps is changed. Further, in some cases, temporary storage of a wafer is required during the waiting period between one step and the next. In such a case, the wafer must be stored according to a method suitable for the condition thereof.
For example, if there is a waiting period between the polishing step and the subsequent cleaning step, a wafer may be stored in water during the waiting period. The reason is that if the wafer is allowed to stand in the air, polishing slurry dries and adheres to the wafer. The adhered slurry is difficult to remove in the subsequent cleaning step.
In such a case where a wafer is stored in storage water, a surfactant is sometimes added to the storage water in order to improve the particle-removing performance of the storage water. In this case, in order to maintain a constant concentration of the surfactant, the wafer is usually stored while being immersed in water contained in a container (hereinafter the water may be called “pit water”).
Generally, ultrapure water is used as such water for storing a silicon wafer, so as not to contaminate the wafer.
It is known that if heat treatment is performed on a wafer without removal of impurities, especially heavy metals, adhering thereto, the impurities adversely affect the electrical characteristics of semiconductor devices. For this reason, a cleaning step for removing impurities is usually provided before the heat treatment.
Therefore, detecting and removing contamination of cleaning solution and that of the surface of a silicon wafer after the cleaning step are highly important, and conventionally, techniques for removing such contaminant have been studied.
However, the conventionally studied techniques have been directed toward removing substances which have adhered to a wafer in steps preceding the cleaning step. Specifically, in the conventional techniques, contaminants which have adhered to a wafer in steps preceding the cleaning step are removed through cleaning, or contamination during the cleaning is prevented, to thereby yield a silicon wafer having a clean surface.
However, even when cleaning is performed, while the concentration of contaminant in cleaning solution is controlled, in order to produce a silicon water having a clean surface, a final wafer that has undergone all processing steps sometimes becomes defective in that its oxide dielectric breakdown voltage is degraded.
Degradation of oxide dielectric breakdown voltage is generally known to occur when a wafer is contaminated with a metal and the metal remains on the wafer surface.
The present inventors analyzed metallic impurities remaining on the surface of a silicon wafer after cleaning, and performed an investigation in order to find a correlation between the concentration of impurities and the number of defective wafers suffering degradation of oxide dielectric breakdown voltage. As a result, they found that the concentration of impurities is not a factor that determines whether or not a wafer becomes defective. That is, they found that even in the case of a defective wafer having degraded oxide dielectric breakdown voltage, proper cleaning had been performed and therefore impurities had been removed.
In view of the foregoing, the present inventors analyzed the causes of such degradation of oxide dielectric breakdown voltage, and found that such degradation of oxide dielectric breakdown voltage is caused by a conventional method of storing a silicon wafer in a step before the cleaning step. Especially, in the case where storage water contains ions of metals, such as copper and silver, having an ionization tendency lower than that of silicon, if a silicon wafer immediately after being subjected to polishing and having a hydrophobic surface is stored in the storage water, the quality of an oxide film of the wafer degrades significantly during subsequent thermal oxidation, resulting in occurrence of degradation of oxide dielectric breakdown voltage.
This phenomenon can occur in any silicon wafer having a hydrophobic surface, and does not occur only in a wafer immediately after polishing. For example, an epitaxial wafer—which is produced through growth of an epitaxial layer on a silicon wafer substrate—has been confirmed to have a hydrophobic surface after the epitaxial growth and to suffer the same problem.
Further, the results of investigation revealed that especially in the case where the metal impurity is Cu, degradation of oxide dielectric breakdown voltage occurs even when the concentration of Cu ions in storage water is 1 ppb or less.
Especially, the present inventors found that in the case where the concentration of Cu ions in storage water is higher than 0.01 ppb, if a silicon wafer having a hydrophobic surface is stored in the storage water immediately after polishing, the quality of oxide film degrades significantly during a subsequent thermal oxidization step, resulting in occurrence of degradation of oxide dielectric breakdown voltage.
Various factors, such as contaminant carried over from preceding steps, can cause such contamination of storage water at a low level of Cu ions. For this reason, there has existed demand for a technique for prevention of degradation of oxide dielectric breakdown voltage, which would otherwise occur due to the above-mentioned causes.